Geosynchronous ("geo") satellites were first proposed by Arthur C. Clarke many years ago for use with communication systems. Communication systems include television, two way communications, surveillance equipment, weather monitoring equipment and other similar equipment. geo satellites operate based on the physical concept that a satellite, at the proper working radius, orbits the earth at the same angular velocity as the earth's rotation. These satellites therefore appear to be fixed relative to a point on the earth.
This arrangement allows an antenna on the earth to continually point at the satellite. This facilitates use of the geosynchronous satellites for communications applications.
The inventors of the present invention have noted a number of drawbacks associated with geosynchronous ("geo") satellite systems. One major drawback is the cost to raise a satellite into a geo orbit. Geosynchronous orbit occurs at around 36,000 kilometers. The cost to boost the satellite into orbit is directly proportional to the height of the orbit. Therefore, it is expensive to boost a satellite into geosynchronous orbit. This cost must be amortized over the lifetime of the satellite, making geo satellites very expensive.
Another problem results from the geometry of coverage of a geosynchronous satellite system. A three satellite geostationary satellite system could have the satellites spaced equally along the equator, at 120.degree. intervals. Their limit of visibility on the equator is calculated from the relationship: EQU 2{cos.sup.-1 (R.sub.E /a.sub.geo)}=2{cos.sup.-1 (6378/35786)= EQU 2{79.73 deg}=159.47 deg,
where 6378 is the radius of the earth in kilometers, and 35786 is the radius out to the geostationary ring. Taking difference between the above value and 120 degrees, it is clear that there is approximately 40 degrees of overlapping coverage by two adjacent geo satellites for an observer on the equator. There will be even less at greater latitudes.
Many global services, however, require world-wide transmission of their information to the whole world. Since each of the satellites only covers one part of the world, some other way must be used to disseminate the information from the source to the satellites covering the rest of the world.
The information begins its transmission at a link. That link transmits up to the satellite in orbit, which then retransmits the information to communicate to, or "cover" one portion of the earth. The same information must also be transmitted to another of the satellites to cover another part of the earth. The information is either sent: 1) over a land line between the link on the earth and ground stations that service areas for the other satellite(s), or 2) via satellite-to-satellite transmission. The land link requires additional equipment and expense. The satellite link also requires additional equipment, but in addition operates a transmission across the two ends of the 42,000 kilometer equilateral triangle. This requires a transmission which is some 70,000 kilometers long. This system requires a second antenna on each of the satellites in addition to complicating control and pointing structure. Even then, the long communication channel may cause noise in the channel.
One of the most difficult-to-solve problem results from the geometry of the geosynchronous orbit. There is only one available orbital position ("band") for geosynchronous satellites. This band is already saturated with satellites. Satellites occupy the geo band with only 2.degree. of spacing therebetween. These are referred to as orbital "slots". Most of the slots are now occupied, making it difficult to find positions for any more geostationary satellites. However, other satellite locations cannot be allowed to interfere with the communication to the geo satellites when operating at the same frequencies.
The system of the present invention obtains the advantages of geosynchronous satellites without using the high altitude circular orbit normally used for geo satellites. The present invention uses a plurality of satellites in orbits chosen such that each desired point of coverage on the earth communicates with a different satellite at different times, and in a direction of antenna pointing separated angularly from any geo satellite(s), such that there is no radio frequency interference, even when operating at the same frequency as a geo satellite. Thus, the present invention alleviates the present "geo-slot" problem. The lower altitudes of the present invention also lead to smaller link distances from ground-to-satellite and from satellite-to-satellite, decreasing the power required due to path loss. These lower altitudes also decrease the time delay which can be annoying in voice transmissions. Thus, the present invention provides a unique solution to some of the problems of using geo satellites.